This application is based on applications No. 2000-102938 and 2000-136540 filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
This invention relates to a fixing device used in an image forming apparatus such as electrophotographic copying machine, a printer and the like and a fixing rotatable member therefor, that is, a heating roller and a pressure roller.
2. Description of the Related Arts
In the electrophotographic image forming apparatus such as a copying machine, a printer, a facsimile and the like, light according to an image is applied to a charged photosensitive member to form an electrostatic latent image according to the image. Toner is electrostatically adsorbed to the electrostatic latent image to be developed. The developed toner image is electrostatically transferred and adsorbed to a recording medium (or once to a toner image carrier, and then from the toner image carrier onto a recording medium). When the recording medium to which the toner image is electrostatically adsorbed passes through a fixing device, normally heat and pressure are applied to the toner image to be fixed to the recording medium. This electrophotographic image forming apparatus is well known, so the further description will be omitted.
FIGS. 1, 2 and 3 show an example of a fixing device for explaining the prior art and the embodiments of the invention. FIG. 1 is a sectional view for giving an outline of a fixing device, and FIGS. 2 and 3 are respectively a partial sectional view of a heating roller and a partial sectional view of a pressure roller, which are used in the fixing device of FIG. 1.
The fixing device 1 has a heating roller 2 and a pressure roller 3, and the pressure roller 3 has a cylindrical core member 31 made of rigid material such as metal or the like and a thin silicone rubber surface layer 32 on the outer surface thereof. The pressure roller 3 is pressed toward the heating roller 2 by an energizing member such as a spring 6 or the like. A recording medium 4 to which toner 5 is electrostatically adsorbed is transported in the direction of an arrow (c) from the left in the drawing. The heating roller 2 has a cylindrical core member made of metal such as aluminum and an elastic layer 9 on the periphery thereof. The interior of the core member 8 is provided with a heater 7, and the heating roller 2 is driven to rotate in the direction of an arrow (a) by a driving source not shown. The pressure roller 3 is rotated in the direction of an arrow (b), following the rotation of the heating roller 2.
When the heating roller 2 is subjected to the pressure of the pressure roller 3, the elastic layer 9 is mainly deformed to form a nip part (n) having a width. When the transported recording medium 4 passes through the nip part (n), the recording medium 4 is subjected to sufficient heat and pressure, so that the melted toner 5 enters between fibers of the recording medium 4.
The elastic layer 9 of the heating roller 2 is, as shown in FIG. 2, formed by three layers. The lowermost layer 91 coming into contact with the aluminum core member 8 is made of comparatively thick silicone rubber. An intermediate layer 92 coming into contact with the lowermost layer 91 has oil resistance, and is made of fluoro-rubber, for example. The intermediate layer 92 may be further formed by plural layers. A surface layer 93 coming into contact with the intermediate layer 92 is made of comparatively thin silicone rubber impregnated with silicone oil, and positioned on the outermost side of the heating roller 2.
The silicone rubber used as material of the surface layer 93 of the heating roller 2 and the surface layer 32 of the pressure roller 3 normally presents insulating property with the surface resistivity of 1016xcexa9 or higher. Therefore, the surface layers 93 and 32 show a very large negative charging characteristic at the time of coming into contact with and separating from the recording medium 4 to generate static electricity.
When the recording medium 4 passes through the fixing device 1, the recording medium is strongly sucked to the heating roller 2 and the pressure roller 3 by the generated static electricity. Consequently, encountered is the problem that the recording medium 4 is curled around the rollers 2,3 to cause a jam.
A known technique for solving the problem is such that a conductive filler is added to the lowermost layer 91 of the heating roller 2 or the surface layer 32 of the pressure roller 3, whereby the surface resistivity of the silicone rubber is adjusted to 1012xcexa9 or lower so that the generated charges are released to the core member 8 or 31 to prevent curling of the recording medium 4.
The surface layer 93 of the heating roller 2 directly comes into contact with toner on the recording medium 4. Though it is desirable that all of toner is fixed on the recording medium 4 by fixing, actually some of toner transfers and adheres to the surface layer 93 of the heating roller 2. This is the so-called toner offset phenomenon. To prevent this toner offset, a lubricant is further added to the surface layer 93. In the color copying machine and the color printer, the surface lubricating performance is especially thought important, so a comparatively large amount of a lubricant is added.
Further, as the surface layer 93 of the heating roller 2 and the surface layer 32 of the pressure roller 3 are repeatedly deformed and loosened, and further heated to a high temperature, they are rapidly deteriorated. An additive for preventing such deterioration is further added to the surface layers 93 and 32.
Since these kinds of additives have a tendency of mutually restraining individual properties, and from a viewpoint of keeping up strength and elasticity, a large quantity of additives can not be added to silicone rubber of the surface layers 93 and 32, it is very difficult to select the quantity of the above additives of many kinds and the combination of kinds.
This invention has been made to overcome the above disadvantages and provides a fixing rotatable member improved in conductive property, surface lubricating property, curling preventing performance and deterioration resistance to be well-balanced. Further, the invention provides a heat fixing device using an improved fixing rotatable member.
In the specification of this invention , the residual elongation percentage is defined as follows. A sample having an original length LO is elongated by applying force (tensile stress) to the sample so that a length of the elongated sample becomes 1.5 times as long as the original length LO. After that, when the applied force is made zero, the sample shrinks to reach a length L. The residual elongation percentage is (Lxe2x88x92LO)/L0, and expressed by %.
The fixing rotatable member used in the heating fixing device of the invention includes a base member and a surface layer, and the surface layer has a surface resistivity of 1xc3x971016xcexa9 or lower and a residual elongation percentage of 3% or lower. The surface layer may contain dimethyl silicone rubber or phenyl denatured silicone rubber as elastic material. Further, wet-type silica may be contained in the surface layer. Further, silicone oil may be contained in the surface layer. Further, the base member may be formed by the metallic core member and a first elastic layer on the outside thereof. Further, silicone rubber or fluorine containing rubber may be used in the first elastic layer. Further, the base member has a second elastic layer on the outside of the first elastic layer, and the second elastic layer may be formed of fluorine containing rubber.
Other objects and advantages besides those discussed above shall be apparent to those skilled in the air from the description of a preferred embodiment of the invention which follows. In the description, reference is made to accompanying drawings, which form a part thereof, and which illustrate an example of the invention. Such example, however, is not exhaustive of various embodiments of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention.